Since light-emitting diodes (LEDs) offer increased functionality and efficiency compared to many other lighting technologies, LEDs are being used for an increasing number of lighting applications. For many lighting applications it is desirable to electrically connect LEDs with other electronic devices, including in some cases other LEDs. For example, when LEDs are used as light sources for illumination, it is convenient to power the LEDs with AC power supplies of voltages of 100 V or greater. Since LEDs are inherently low voltage DC devices, high voltage AC power cannot directly power an LED unless the LED is connected to a circuit that converts the AC power to low voltage DC power.
It has recently been discovered that LEDs can be directly powered by high voltage AC power by connecting the AC power to several interconnected LEDs. FIG. 1A illustrates an exemplary LED device that may be directly driven by a three-phase AC power source. An equivalent circuit diagram of the device in FIG. 1A is shown in FIG. 1B. FIG. 1B shows that the circuit contains six groups of paired LEDs; the groups being designated C11, C21, C12, C32, C13, and C33. Each group contains a pair of LEDs connected in parallel and oriented so that when one LED is forward biased, the other LED is reverse biased. The term anti-parallel will be used to refer to the arrangement of the LEDs in each group (i.e. connected in parallel and oppositely oriented). The circuit further includes paired LEDs C22, C23, and C31. The three-phase power is connected to contact points P1, P2, and P3.
The arrangement of LEDs in the circuit eliminates the need for a drive circuit. A voltage applied across serially connected LEDs will be distributed across the LEDs so that each LED sees only a fraction of the total voltage. Thus the serial connection between the various LED groups lowers the high voltage applied at the contact points so that each of the individual LEDs is subjected to a lower voltage. The circuit in FIG. 1B can continuously produce light from AC power because the anti-parallel arrangement of the LEDs in each of the groups ensures that one of the LEDs in each group will produce light regardless of the polarity of the power applied to the pair, and the distribution of the groups among the contact points ensures that at least one of the pairs of groups will be subjected to a non-zero voltage from one of the three phases. Thus the circuit shown in FIG. 1B allows a high AC drive voltage to be used to drive LEDs that otherwise will not be able to stand the high voltage without an additional drive circuit.
The device in FIG. 1A contains the circuit from FIG. 1B monolithically formed on a single substrate. The six LED groups C11, C21, C12, C32, C13, and C33 and the three power contact points P1, P2, and P3 are disposed on a single surface of substrate SUB. The device shown in FIG. 1A suffers from drawbacks. FIG. 1A illustrates that in order to accommodate the circuit, a relatively large area substrate SUB is needed. Therefore, it is hard to form a compact AC-powered LED device if the connection scheme as shown in FIG. 1A is adopted. The situation is further worsened when far more than two LED groups are connected in series in order to distribute the voltage of a higher voltage AC power source.
Accordingly, what is needed in the art is an improved method of interconnecting a plurality of LEDs.